Orbital power tools such as sanders and polishers generally include a pad that is normally adapted to support an abrasive element such as sanding paper. The pad is coupled by a transmission means to a motor arranged in a housing. The transmission means can incorporate a cam rotationally driven by the rotary drive shaft. The cam is housed in a circular aperture that is positioned in the centre of the pad. The rotation of the cam drives every point of the pad in a circular orbit whose radius equals the eccentricity of the cam ie the distance between the rotary axis of the rotary drive shaft and the centre of the circular aperture which is substantially coincident with the centre of the pad. By allowing the pad to rotate around the centre of the circular orbit, it describes a combined rotary/orbital motion referred to as a “random orbit”.
The orbital motion can be envisaged as a linear motion (or stroke) in which the pad mass is accelerated in a certain direction. The acceleration produces a reaction force directed in the opposite direction. This reaction force manifests itself as an unwanted vibration which is transmitted to the housing and ultimately to the operator's hand and arm. The amplitude of this unwanted vibration depends on the diameter of the orbit and on the ratio between the mass of the pad and the mass of the tool.
In order to keep vibrations beneath an acceptable level, conventional tools are designed in such a way that the working surface of the pad and the orbital diameter are relatively small. However, these limitations reduce the efficiency of the machine. In order to compensate for these limitations in efficiency, operators frequently apply a certain pressure or load to the tool in order to increase the friction on the work piece with the result that vibrations are amplified. In order to counteract the resulting increase in vibrations, the operator tends to grasp and apply the tool with even more force to the work piece. By doing this, the effective mass of the machine is increased and the vibrations are absorbed by the operator's hand and arm with often severe consequences for the operator's health. For example, even low usage operators may experience numbness and tingling in their fingers, hand and arm within a few minutes of operation and this may be lead to an unpleasant loss of feeling and control in the fingers that can last for hours after use has ceased. If use is prolonged for hours, a full recovery can take several days. The consequences for professional workers can be even more severe and long term may lead to retirement and high social costs. On the other hand, adopting strict guidelines relating to vibration threshold values would have a severe impact on productivity and costs.
Operators of power tools tend to apply a certain load to the tool so that the speed of the work is increased. The increase in the working efficiency that is achieved by the increased load is due exclusively to the increase in friction between the pad and the work piece. On the other hand, the increased load unbalances the tool and increases the unwanted vibrations. The diameter of the unwanted vibrations is subtracted from the orbital diameter of the pad. In practice, the effective working orbital diameter is the result of the theoretical orbit diameter less that of the unwanted vibrations.
An arrangement for overcoming the above-mentioned drawbacks adopts one or more counterbalances (eg eccentric masses or counterweights) that move in a direction opposite to that of the pad to counterbalance the vibrations. Examples of this kind of arrangement are illustrated in U.S. Pat. No. 4,660,329, U.S. Pat. No. 4,729,194, U.S. Pat. No. 5,888,128, U.S. Pat. No. 6,244,943, US-A-6206771, U.S. 2001/0003087, DE-A-3922522, EP-A-303955, EP-A-0455618, WO-A-98/01733 and WO-A-02/068151. In general, this type of arrangement works satisfactorily when the pad is not touching the work piece but displays major limitations in normal use. As soon as the pad is placed on the work piece, the load effectively modifies the mass of the pad and the ratio between the mass of the pad and the mass of the counterbalance is altered. As a result, the counterbalance fails to eliminate the vibrations induced by the heightened effective mass of the pad. The higher the load, the greater the system imbalance and the higher the level of unwanted vibrations. With a load tending to infinity, the pad will be at a standstill and the tool will vibrate with an amplitude equal to the radius of the orbit of the pad.
Another arrangement for overcoming the above-mentioned drawbacks uses elastic materials as an interface between the tool and the operator's hands for dampening vibrations. The kinetic energy of the vibrations is converted into thermal energy. Examples of this type of arrangement are illustrated in U.S. Pat. No. 4,905,772, U.S. Pat. No. 5,453,577, U.S. Pat. No. 5,347,764, U.S. 2001/0011856 A1, WO-A-03/049902. However, by interposing an elastic element between the housing and the operator's hand, the tool is free to vibrate with greater amplitude than if it was firmly held by the operator. In practice, the operator instinctively feels the decreased efficiency of the machine and tends to grasp it with increased force in an attempt to restore efficiency. By doing this, the effeciency of the elastic element is minimized so that vibrations are transmitted to the operator's hand and arm. Moreover, the increased muscular force reduces the human body's natural capability to dampen vibrations.